Many gazelles lived along the riverbanks of the African savanna.
Although the environments on both sides of the river were similar, the gazelles on the east bank had greater reproductive ability and ran faster than those on the west bank.
A zoologist who noticed this caught ten gazelles from each side and moved them to the opposite bank.
A year later, the gazelles that had been moved to the west had multiplied to fourteen. In contrast, the gazelles moved to the east were mostly eaten by wolves—only three survived.
The reason the gazelles on the east bank were stronger was because a pack of wolves lived nearby.
Without natural predators, the gazelles on the west bank had grown weak.